Lecanora campestris

1 April 2025

TAXON:	Lecanora campestris (Schaer.) Hue (1888)
RECENT SYNONYMS:	Lecanora campestris subsp. dolomitica O.L. Gilbert (1984)
FAMILY:	Lecanoraceae
GROWTH FORM:	Crustose, heteromerous
SUBSTRATES:	Calcareous and basic rocks, including artificial substrates such as concrete, cement and mortar; nutrient-enriched siliceous substrates; occasionally on highly eutrophicated bark and worked wood
PHOTOBIONT:	Trebouxia alga
REPRODUCTION:	Apothecia, pyncidia; soredia (L. campestris subsp. dolomitica)
ASCUS:	Broadly clavate; apex Lecanora-type; 8 spores
SPORES:	Ascospores ellipsoid, aseptate, 10-14(-17) × 6-8.5(-9) µm; conidia thread-like to curved
NOTABLE FEATURES:	Thallus grey to white, rough-textured and rimose-areolate, often circular; prothallus typically conspicious, white, fimbriate; apothecia lecanorine, small (typically <0.8 mm), sometimes abundant; discs reddish-brown; soralia white, circular, convex (L. campestris subsp. dolomitica)
CHEMICAL TESTS:	Thallus K+ yellow, K/UV+ yellow (atranorin and other substances)
HABITAT:	Lowlands where suitable substrate available
DISTRIBUTION:	Widespread and common in Britain
CONSERVATION STATUS:	Least Concern
LICHENICOLOUS FUNGI:	Lichenoconium reichlingii, Lichenodiplis lecanorae, Muellerella erratica (L. campestris subsp. dolomitica), Muellerella lichenicola, Toninia subfuscae, Vouauxiella verrucosa
IDENTIFICATION DIFFICULTY:	Green 2: Field identification possible with care
CONFUSION SPECIES:	Lecanora caesiosora (with L. campestris subsp. dolomitica), L. horiza

FIELD NOTES

Lecanora campestris is a strong contender for the most common Lecanora species in Britain. It’s almost certainly the most common saxicolous species. No doubt you’ll find it near you on natural or worked stone, or on hard artificial substrates such as concrete. It tolerates disturbance well, so look for it even on the pavement under your feet.

L. campestris is one of the ‘Lecanora subfusca-group’ of lichens. Like others in the group, it has the classic Lecanora ‘look’: pale thallus, and apothecia resembling jam-tarts with ‘pastry-crust’ rims and reddish-tinged ‘filling’. It can be readily distinguished from other subfusca-group lichens by a combination of characters: saxicolous habit, preference for basic or nutrient-enriched stone, small apothecia, and usually obvious white prothallus.

The less common L. horiza is the one subfusca-group species that presents a real potential confusion with L. campestris. It’s typically corticolous, but it can be saxicolous as well. However, L. horiza tends to have comparatively larger apothecia with darker and shinier discs, and with a more constricted base that makes them appear to be ‘popping out’ of the thallus. It also tends to lack any obvious prothallus.

Such subtleties can be hard to observe in the field, though, especially if you’re unfamiliar with L. campestris. What’s more, very well-developed or ‘exuberant’ L. campestris can feature apothecia recalling those of L. horiza. Indeed, it wasn’t realised until relatively recently that L. horiza even existed on stone in Britain. Previously, British field lichenologists recorded all such saxicolous Lecanora as L. campestris (see Maliček & Powell 2013).

It’s said that L. campestris prefers horizontal stone surfaces, while L. horiza prefers vertical surfaces. This distinction may hold, but it doesn’t separate the species. L. campestris frequently colonises vertical surfaces, such as the sides of gravestones. It seems perhaps more accurate to say that L. horiza prefers vertical surfaces, than to say that L. campestris has a markedly distinct preference for horizontal ones.

According to the official British lichen flora (see link to LGBI3 above), L. horiza fluoresces UV+ pale orange, while L. campestris doesn’t fluoresce. I haven’t myself observed this distinction, and I don’t find it repeated in other European or North American sources. Rather, it seems sometimes to be suggested that L. campestris can fluoresce UV± pale orange!

Fortunately, typical L. campestris is easy to recognise once you’ve seen it. It also tends to occur in abundance. So you’re more likely to be confident that you have at least some L. campestris on the surface you’re examining, than you are to fret that you can’t distinguish any of it from L. horiza.

As a final complication, there exists a sorediate form of L. campestris, known as L. campestris subsp. dolomitica. But it appears to be entirely restricted to the North Midlands of England, and mostly to local dolomite outcrops. In this sorediate form, it might be confused with L. caesiosora, but it lacks the brown granules in its thallus that characterise that species.

Oliver Gilbert, who discovered L. campestris subsp. dolomitica, awarded it subspecies rank because he found no intermediates between the sorediate and apotheciate forms (see Gilbert 1984). It’s since become clear, however, that the same L. campestris thallus can in fact produce both soredia and apothecia, calling into question whether the two forms are sufficiently different genetically. Yet sorediate forms have also been found that appear to be parasitising apotheciate forms! Lichens are never so simple.

LAB NOTES

Lecanora campestris can normally be identified without microscopy. Sadly, no microscopic features distinguish it from L. horiza. Similar ‘subfusca-group’ lichens can often be separated by the presence and placement of POL+ crystals in the apothecium. But L. campestris and L. horiza both accumulate small, dispersed POL+ calcium oxalate crystals in the amphithecium, and both lack any crystalline granules in the epithecium.

SPECIMENS

Lecanora campestris subsp. campestris : England : VC25 East Suffolk : TL3389 : April 2025 : On brick wall coping